The present invention relates to a process for producing a metal plated steel tube, comprising a thermal spraying step to the internal or external surface of the steel tube.
Traditionally, processes for producing metal plated steel tubes are known which comprise a step of continuously forming a steel plate that is plated on both sides with a heterogeneous metal (typically, zinc) into a tubular shape using roll forming, a step of continuously welding the tubular plated steel plate so formed at its end faces to form a tube, a step of continuously cutting weld beads that are formed on the external surface of the steel tube during the previous step and a step of thermal spraying portions where part of the plated layer has been removed due to the cutting with the same metal as the one for the plating or with a heterogeneous metal (Patent Reference 1).
In such steps, it is also contemplated that the final thermal spray coating of the unplated portions may be substituted with continuous molten plating (Japanese Patent Application No. 2002-225668 by the Applicant (unpublished)). Patent Reference 1: Japanese Unexamined Patent Publication No. 1993-148607.
In such a field of art, thermal spraying is exclusively used for repair applications. It means that the idea of thermal spraying metal tubes partially with a metal on their unplated portions in order to shield such portions and the like from the ambient atmosphere exists. Such an idea that, in order to modify a whole plated layer, thermal spraying a whole tube instead of specific portions thereof to form a metal layer all over the tube does not, however, exist.
On the other hand, for the purpose of modifying a whole plated layer instead of repairing, steps are known in which molten plating is also applied using a similar or dissimilar metal to the steel tube produced according to the steps for production described above. For example, it is known that by incorporating aluminum to zinc plating, corrosion resistance that may not be obtained with zinc alone may be realized. As environments where products are used become increasingly stringent year after year, with lengthening of high corrosion resistance and service life being desired, there is a need for new alloy plating. As such, characteristics required include wear resistance, heat resistance, electrical insulation, electromagnetic shielding and electrical conductivity, in addition to corrosion resistance, all of which may be realized by coating with other metallic and nonmetallic materials in a composite manner.
Conventionally, the step of coating with multiple heterogeneous metals involves forming a steel plate plated with zinc on both sides using roll forming and the like, followed by continuous welding and removal of beads and the like, before molten plating with zinc or a different metal (aluminum, for example). However, providing anew a molten plating bath along an existing line will present a difficulty in terms of process design and arrangement and initial investment will be substantial.